The present invention relates to a circuit for terminating a bus line or network in a precision impedance and, more particularly, to providing termination for a small computer system interface (SCSI) to which SCSI peripheral devices are coupled through a cable bus in order to minimize line reflection.
There are many circuit applications comprised of a bus network or line in which the bus must be terminated in a proper "Thevenen" termination. For example, computers and computer peripheral devices such as printers, modems and the like can be networked or daisy chained together using a cable or bus connected to the respective SCSI outputs of the devices and the computers wherein all signals are common between the SCSI devices. In order to reduce or inhibit induced signal reflection onto the cable, it is important that both ends of the cable or bus be terminated in its characteristic impedance. Typically, two of the daisy chained SCSI devices in the network must present a matched termination impedance to the cable bus, those being the devices at the extreme ends of the bus network. In the industry, as set by the Accredited Standards Committee, this termination impedance is provided by a pair of resistors series coupled between a power supply conductor and logic ground with the interconnection therebetween being coupled to the output of the terminator circuit to the bus. The value of the resistors are specified to be 220 and 330 ohms respectively in order to provide an effective impedance of 132 ohms to closely match the characteristic impedance of the connecting cable.
Presently, most SCSI systems use a terminator network of discrete components which are made to be plugged into a socket of the terminating computer related device acting as the termination unit. To connect the termination network to a particular peripheral device, an appropriate resistor array is plugged into a provided socket. If no termination network is required in a particular device, the resistor array is either omitted or must be removed (if another device in the network already has a terminator circuit inserted therein). However, if the peripheral device including the terminator circuit is removed from the network then a remaining peripheral device must have a terminator circuit inserted therein to insure minimum line reflection. This need to manually insert and/or remove the resistor array network is highly undesirable.
Therefore, a need exists for providing a terminator circuit that can be permanently inserted in SCSI devices and which can be activated or deactivated by the device to either switch the circuit onto the bus or not respectively as required to provide the impedance matching.
In addition, it is desirable to fabricate such a terminator circuit in integrated circuit form while maintaining resistor tolerances to .+-.5%.